Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb the unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is normally attached to the unsprung portion of the vehicle. The piston is normally attached to the sprung portion of the vehicle through a piston rod which extends through the pressure tube. The piston divides the pressure tube into an upper working chamber and a lower working chamber both of which are typically filled with a hydraulic liquid. Because the piston is able, through valving, to limit the flow of the hydraulic liquid between the upper and the lower working chambers when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibration which would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle. In a dual tube shock absorber, a fluid reservoir or reserve chamber is defined between the pressure tube and a reserve tube. A base valve is located between the lower working chamber and the reserve chamber to also produce a damping force which counteracts the vibrations which would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle.
Shock absorbers filled with a hydraulic liquid have met with continuous success throughout the automotive industry. While meeting with success in the automotive industry, hydraulic liquid filled shock absorbers are not without their problems. One problem with these prior art shock absorbers is the difficulty to make them sensitive to the frequency of the vibrations. Complex systems have been developed to produce hydraulic liquid filled shock absorbers which are relatively soft for high frequency vibrations while being relatively stiff for low frequency vibrations. Other problems associated with the prior art hydraulic liquid filled shock absorbers include the variability in their damping forces due to the temperature changes of the hydraulic liquid. As the temperature of the hydraulic liquid changes, the viscosity of the liquid also changes which significantly affects the damping force characteristics of the liquid. In addition, any aeration of the hydraulic liquid during operation adversely affects the operation of the damper due to the introduction of a compressible gas into a non-compressible hydraulic liquid. Finally, the hydraulic liquid adds to the weight of the shock absorber as well as presenting environmental concerns regarding the use of a hydraulic liquid.
In order to overcome some of the problems with shock absorbers which utilize hydraulic liquid as the damping medium, shock absorbers have been developed which utilize a gas such as air as the damping medium. The use of a gas as the damping medium produces a frequency dependent damper which is less sensitive to temperature changes when compared to hydraulic liquid dampers, it is not adversely affected by aeration over time, it is lower in weight and it is environmental friendly due to the elimination of the hydraulic liquid.
While gas filled shock absorbers have addressed some of the problems associated with hydraulic liquid dampers, they are not without their own unique problems. One area that presents problems for the gas filled shock absorbers is the sealing system incorporated into the rod guide assembly which seals and lubricates the piston rod. In most prior art gas springs or gas filled shock absorbers, the sealing system incorporated into the rod guide assembly consists of two seals with an oil chamber positioned between them. The oil provides lubrication for the seals and it forms an extra barrier to the gas molecules in order to guarantee a perfect sealing. A problem with this prior art design is that the pressure in the oil chamber can increase over time. The pressure is at atmospheric pressure when the shock absorber is manufactured and it increases up to the static pressure of the working chamber or even worse, up to the highest dynamic pressure occurring in the working chamber over time. Such a pressure raise causes an increase in the amount of seal friction and thus the accelerated wear of the seals. The cause of this increase in pressure is the migration of gas molecules through the seal from the high pressure side (working chamber) to the low pressure side (oil chamber).
The continued development of gas filled shock absorbers has been the development of improved sealing system for the rod guide assembly which address the above described issues.